The invention relates to a fixing device by means of which an aircraft propulsion system comprising an engine and a pod is connected to a strut fixed to a structural element of the aircraft such as a fuselage or wing element.
The invention also relates to an attachment strut able to support an aircraft propulsion system through such a fixing device.
The fixing device and the strut according to the invention can be used on any type of aircraft. A preferred application relates to aircraft of modern design, whose engines are equipped with very large diameter fans.
On an aircraft, the strut constitutes the connecting interface between the propulsion system, including the engine and the pod, and the aircraft fuselage or wing. It permits the transmission to the aircraft structure of forces generated by the engine (structural function). It also ensures the passage of fuel, electricity (control and power), hydraulics and air between the propulsion system and the aircraft (system function). Apart from these two functions, the strut must respect different constraints such as the obtaining of maximum safety, with an aerodynamic drag, a weight and a cost which must be as low as possible.
To ensure the transmission of forces, the strut comprises a primary structure, having a frame, e.g. in box form. In this case, said frame comprises ribs and panels, as well as mounts through which the strut is connected on the one hand to the structure of the aircraft and on the other to the propulsion system.
The strut also comprises a secondary structure ensuring the segregation and retention of the systems, whilst supporting aerodynamic shrouds.
In order to be able to ensure the transmission of forces between the propulsion system and the aircraft structure, the mounts interposed between the strut and the propulsion system are at least partly anchored to the central casing. Therefore the strut penetrates the secondary flow duct formed between said central casing and the pod which surrounds it. To disturb to the very minimum the air flow in said secondary duct, the front part of the strut must be as narrow as possible.
As is very diagrammatically illustrated in FIGS. 1A and 1B of the attached drawings, there are at present two main types of devices for fixing a propulsion system 1 to a not shown strut fixed to a structural element of an aircraft.
A first type of known fixing device, illustrated in FIG. 1A, is generally known as a xe2x80x9ccore fixturexe2x80x9d. This fixture is characterized by the use of a front mount 3 and a rear mount 4, directly linking the strut to the central casing 5. The front mount 3 links the strut to a front part of the central casing 5, located just to the rear of the fan stator case 6. Said front part of the central casing 5 mainly constitutes the casing of the high pressure compressor of the engine. The rear mount 4 is interposed between the strut and the rear of the central casing 5.
To facilitate understanding, an orthonormal fix OXYZ is allocated to the propulsion system 1 and in it the longitudinal axis OX coincides with the longitudinal axis of the propulsion system 1 and is oriented towards the front. The lateral axis OY is perpendicular to the OX axis and to the median plane of the strut (said latter plane being vertical or perpendicular to the lower surface of the wing when the engine is suspended on said wing in the manner shown). Finally, the OZ axis is perpendicular to the OX and OY axes, i.e. vertical in the embodiment shown. The OZ axis is oriented from the engine to the strut, i.e. upwards. In the case of an engine attached laterally to the fuselage of an aircraft, the OY axis would be oriented downwards and the OZ axis in a substantially horizontal plane. However, the OY and OZ axes will be respectively called the xe2x80x9clateral axisxe2x80x9d and xe2x80x9cvertical axisxe2x80x9d throughout the text.
In a fixture of the xe2x80x9ccorexe2x80x9d type, as illustrated in FIG. 1A, the front mount 3 ensures the transmission of forces exerted between the central casing 5 of the engine and the strut in longitudinal X, lateral Y and vertical Z directions (in the case represented of a propulsion system beneath the wing) relative to the propulsion system 1.
For its part, the rear mount 4 ensures the transmission of forces exerted between the central casing 5 of the engine and the strut in lateral Y and vertical Z directions, as well as the transmission of the moment MX in accordance with the longitudinal axis OX.
In the second type of conventional fixture illustrated in FIG. 1B and generally known as the xe2x80x9chybrid fan fixturexe2x80x9d, the link between the propulsion system 1 and the strut is also ensured by a front mount 3xe2x80x2 and a rear mount 4 (cf. also EP-A-741 074 and EP-A-805 108).
The front mount 3xe2x80x2 is interposed between the strut and the fan stator case 6 of the propulsion system 1. It ensures the transmission of forces in the lateral direction Y and vertical direction Z with respect to the propulsion system 1.
As in the core-type fixture, the rear mount 4 is interposed between the strut and the rear part of the central casing 5. This rear mount 4 ensures the transmission of forces exerted between the central casing 5 of the engine and the strut in the lateral direction Y and vertical direction Z with respect to the propulsion system 1, as well as the transmission of moments MX in the longitudinal axis OX. Moreover, two rods 7 linking the rear mount 4 to the front part of the central casing 5 enable the rear mount 4 to also transmit forces exerted between the central casing 5 of the engine and the strut in the longitudinal direction X.
To make aircraft engines more economic, aircraft manufacturers attempt to increase their bypass ratio. This leads to increasing the diameter of the fan, which is generally located at the front of the propulsion system. However, this engine size increase leads to numerous problems associated with existing fixture devices.
Thus, when using a core-type fixing device, as illustrated in FIG. 1A, the diameter difference between the fan stator case and the central casing of the engine increases engine bending phenomena, which are particularly sensitive with this fixture type. Particularly under certain flight conditions and in particular on take-off, the aerodynamic support on the air intake, transmitted on the front of the engine fan, gives rise to a significant engine bending between its two mounts 3 and 4. To prevent the rubbing of the rotary fan blades on the fan stator case 6 and the rubbing of the rotary compressor and turbine blades on the central casing of the engine, it is consequently necessary to provide a clearance between the end of the different blades and the corresponding casings. These clearances increase in size with the rise in the bypass ratio of the engines. Under certain flight conditions and in particular in the cruising phase, the engine resumes its normal bending state. Thus, the clearance at the end of the blades increases with the bypass ratio, so that the overall engine efficiency is reduced.
When the link between the propulsion system and the strut is ensured by a hybrid fan-type fixing device, as illustrated in FIG. 1B, the increase in the diameter of the fan increases aircraft resonance problems, which are particularly sensitive with this fixture type. Thus, this fixture is characterized by the fact that the system constituted by the strut and the engine behaves like a pendulum including a weight (the engine) hung on a wing by a spring (the strut). Under certain flight conditions, the wing excites the thus designed pendulum. To solve this problem, it is not acceptable to increase the hung weight. It is therefore necessary to increase the stiffness of the strut by increasing the thickness of certain of the components forming it. This phenomenon also exists on using core-type fixing devices, but the hybrid fan-type fixture is more prejudicial because it requires a greater weight increase for obtaining the same strut stiffness increase. This problem is accentuated on increasing the engine size. To avoid it, it would be necessary to rigidify the strut, whilst increasing its external dimensions. However, this would mean a significant increase in the strut width, weight, cost and aerodynamic drag, which is clearly undesirable.
The object of the invention is a device for fixing a propulsion system to a strut fixed to a structural element of an aircraft, whose original design enables it to better distribute the forces transmitted through the strut, in order to obviate the problems produced by an increase in engine size and weight.
A further object of the invention is a fixing device, whose original design enables it to reduce engine bending or sagging in the critical flight phases such as take off, so as to optimize engine performance characteristics when cruising.
Yet another object of the invention is a fixing device making it possible to ensure a good vertical and lateral stiffness of the fixture without increasing the dimensions of the strut, so as to eliminate aircraft resonating and vibrating problems without increasing the weight, cost or aerodynamic drag.
According to the invention, these results are obtained by means of a device for fixing an aircraft propulsion system to a strut fixed to a structural element of the aircraft, the propulsion system comprising an engine having a central casing and a fan stator case, the device including a first front mount and a rear mount system, respectively linking to the strut a front part and a rear part of the central casing of the engine, said device being characterized in that it also comprises a second front mount linking the fan stator case to the strut.
The use of at least three unaligned mounts for ensuring the transmission of forces exerted between the engine and the strut makes it possible to considerably reduce the pendulum effect and resonating effect of the aircraft resulting therefrom. This improvement is all the more pronounced as the fan diameter increases.
Moreover, as the forces exerted between the engine and the strut are transmitted thereto at at least three instead of two points, the bending phenomena of the engine and the engine-aircraft links are greatly reduced.
Moreover, the addition of at least one supplementary mount between the engine and the strut permits a better distribution of the forces having to be transmitted to the latter.
Thus, in a preferred embodiment of the invention, the second front mount is normally able to transfer forces mainly exerted in a direction Z oriented radially from the engine to the strut.
Here and in the remainder of the text, the term xe2x80x9cnormallyxe2x80x9d means xe2x80x9cunder normal flight conditionsxe2x80x9d, as opposed to special conditions such as the breaking of a part, landing without wheels down, etc.
The expression xe2x80x9cforces mainly exerted in one directionxe2x80x9d means here and in the remainder of the text that the forces transferred in this way from the engine to the strut are not necessarily perfectly oriented in said direction. Thus, in the case of the Z direction, said forces can be exerted in a direction Zxe2x80x2, which is inclined by a few degrees, e.g. in the OXZ plane, to reduce to the minimum the contribution of forces exerted in this direction to the bending of the engine.
In the preferred embodiment of the invention, the first front mount is normally able to transfer forces mainly exerted in a lateral direction Y with respect to the engine and the rear mount system is normally able to transfer forces exerted in the direction Z oriented radially from the engine to the strut and in the lateral direction Y, as well as a moment along the engine longitudinal axis OX.
In this case, the transmission of thrust forces in a direction X, longitudinal with respect to the engine, can take place normally either through the first front mount, or through the rear mount system, or through a fourth mount interposed between the central casing of the engine and the strut and allocated to said function.
The second front mount is connected to the fan stator case in a structural area thereof able to transmit forces. The first front mount and the rear mount system are connected to the central casing of the engine in structural areas of said casing able to transmit the forces. In the case of the second front mount, said structural area of the fan stator case can in particular be structurally connected to the central casing of the engine by stationary vanes.
The invention also relates to a strut for attaching a propulsion system to an aircraft structural element able to support the propulsion system by a fixing device of the type defined hereinbefore, in which the strut frame comprises a main part, whereof an interface can be connected to the engine casing by the first front mount and by the rear mount system, the strut frame also comprising a front part projecting with respect to the main part, at a location displaced towards the exterior of the propulsion system with respect to said interface, said projecting front part being connectable to the fan stator case by the second front mount.